We are interested in the transcriptional control of T cell development and function. Specifically, we study the gene expression programs that control the choice by intrathymic T cell precursors of the CD4 or CD8 lineage, and perpetuate lineage differentiation in post-thymic T cells. T cells are essential for immune responses. ?Conventional? T cells recognize peptide antigens presented by class I (MHC-I) or class II (MHC-II) classical Major Histocompatibility Complex molecules, and express either of two surface glycoproteins (called coreceptors) that contribute to antigen recognition: CD4, which binds MHC-II, or CD8, which binds MHC-I. Coreceptor expression on mature T cells is mutually exclusive and strongly correlates with both MHC specificity and functional differentiation. That is, the general rule is that MHC I-specific T cells are CD4-CD8+ and cytotoxic (CD8 cells), whereas MHC II-specific T cells are CD4+CD8- and helper or regulatory (CD4 cells). We are interested in the development of CD4 T cells, a key component of adaptive immune responses, which are the target of the human immunodeficiency virus HIV. Our research focuses on the transcriptional control of CD4 T cell development and function, addressing two key questions: how do CD4 cells develop in the thymus and why do they mount helper rather than cytotoxic responses. Two conceptually distinct processes are thought to be involved in the differentiation of CD4 cells: initiating expression of genes specific of the CD4 lineage, referred to as ?specification?, and repressing genes characteristic of the CD8 lineage. We previously identified the transcription factor Thpok, whose expression in T cells is specific of the CD4 lineage, as a major repressor of CD8-lineage gene expression and CD8 T cell differentiation. We are currently examining if and how Thpok contributes to the ?specification? function, by examining the functional responses, in vitro and in vivo, of Thpok-deficient MHC II-restricted cells. In separate studies, we had shown that another transcription factor, Gata3, is required for the expression of Thpok, and is therefore important to specify CD4-lineage gene expression. More recently, we have found that Gata3 contributes to restrain CD8-lineage gene expression in developing CD4-lineage thymocytes. Specifically, enforced expression of Gata3 in developing thymocytes represses expression of the transcription factor Runx3, a critical component of the CD8-lineage specification-commitment program that notably represses expression of CD4. Thus, Gata3 indirectly promotes sustained CD4 expression and thereby CD4-lineage specification. Indeed, an important implication of this line of research is that the contribution of Gata3 to CD4 lineage specification is both direct (e.g. Gata3 promoting expression of Thpok) and indirect (e.g. promoting expression of CD4 by repressing that of Runx3). We are currently exploring the mechanistic bases for this new function of Gata3.Our analyses of the transcriptional network underpinning CD4-lineage ?specification? also rely on high-throughput sequencing and bioinformatics approaches to track the onset of lineage-specific gene expression, and how it depends on Thpok. These analyses, currently in progress, have shown that CD4-CD8 differentiation is accompanied by major epigenetic reconfiguration at lineage specific genes. We have also found that the apparent symmetry between CD4 and CD8 lineage differentiation masks substantial differences in epigenetic modification patterns, notably at the loci encoding CD4 and CD8 genes. We are currently pursuing these analyses focusing on the discovery of new transcripts potentially involved in the lineage differentiation process.